Fix incorrect t2_event_f() usage in performance monitoring doc example

[Copilot]

The code example in docs/api/public-api.md called t2_event_f() with a format string and variadic args, which does not match the function signature — t2_event_f() accepts a single double, not a printf-style format string.

Change

• Replaced the invalid t2_event_f("Slow_Operation", "name=%s,duration_ms=%.2f", operation_name, duration_ms) call with snprintf to compose the marker string, then a correct t2_event_f(marker, duration_ms) call.

// Before (incorrect)
t2_event_f("Slow_Operation",
            "name=%s,duration_ms=%.2f",
            operation_name, duration_ms);

// After (correct)
snprintf(marker, sizeof(marker), "%s_Duration_ms", operation_name);
t2_event_f(marker, duration_ms);

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